Rosin is a well known, commercially available product which is comprised of mainly C.sub.20 fused-ring monocarboxylic acids with abietic type acids being typical components of rosin. Rosin is capable of being subjected to various types of chemical conversions, of which esterification is one of the more commonly performed. The rosin esters which are obtained as a result of esterification are useful in a wide variety of different applications; for example, as tackifiers in hot-melt adhesive compositions and the like. In most applications where rosin esters are employed, it is highly desirable that the rosin ester be as light in color as possible, have a relatively high softening point, and be resistant against oxidation.
Various methods have heretofore been suggested for the esterification of rosin. All the methods which have heretofore been suggested have, however, had one or more substantial disadvantages. Certain of the suggested methods required relatively long reaction times. Other methods caused a substantial increase in the color of the final product as compared to the starting rosin. Still other of the suggested methods required relatively expensive catalyst and often there was a evolution of obnoxious fumes, highly toxic gases, or flammable gases during the esterification reaction.
The method which was traditionally used for the esterification of rosin involved the use of a strong basic catalyst such as calcium oxide to promote the esterification reaction. This method was found to be unsatisfactory in that it required extremely long reaction times and the resulting esterified product was typically highly colored as compared to the starting rosin.
It was also suggested to use aliphatic or aromatic esters of phosphorous acid as the catalyst in the esterification process. The time of reaction using this class of catalyst was substantially decreased as compared to the method employing basic catalysts, and the final product tended to be substantially lighter in color. However, the suggested catalysts were relatively expensive to purchase and during the reaction decomposed causing an evolution of highly obnoxious odors.
It was also suggested to use phosphinic acid as the catalyst for the esterification. Reasonable reaction times were obtained using phosphinic acid as the catalyst, and the resulting product did have a relatively light color. The use of phosphinic acid as the catalyst, however, was found to have a number of inherent and distinct disadvantages. Initially, the cost of the catalyst was relatively high. Since the phosphinic acid catalyst is not recovered from the reaction product and, in fact, appears to decompose during the esterification reaction, the overall manufacturing cost of the rosin esters was increased. An additional problem which was found when using phosphinic acid as the catalyst was that during the esterification process the catalyst apparently decomposed and as a result, phosphine, a deadly toxic gas was released along with other inflammable gases and created considerable manufacturing problems.
What would be highly desirable would be a method for manufacturing rosin esters which could be conducted in a relatively short reaction time, would result in rosin esters having a relatively light color, which would use relatively inexpensive catalysts, and which during the reaction process would not result in the evolution of large quantities of obnoxious, toxic, or flammable gases.